The colours of nature

Pigment-containing chromatophores create the vibrant red and blue hues on the skin of the Siamese fighting fish. When stressed or unwell, their colour can fade. Image credit: endriqstudio via Pixabay

The colours most commonly associated with nature are shades of blues and greens. Very rarely does red make a vibrant appearance, and researchers at the University of Cambridge may have explained why. 

A team from the Department of Chemistry used computational modelling to determine that ‘matt structural colour’ – responsible for the most intense colours seen in nature – is not often found for reds, oranges and yellows. They attribute this to structural limitations which affect the way different wavelengths of light, and therefore different colours, are reflected and absorbed by surfaces. 

To understand what matt structural colour is and how it creates such pure, intense hues, we must first understand how colours in nature are formed. 

Walking through a forest in December, you might find bright red rosehips, but return in early spring and they are shrivelled and browned. This is because the red pigment in the berries has deteriorated. Pigments are coloured proteins, ubiquitous in plants and animals, and are responsible for the tone of a person’s skin (melanin) or the green shades of the leaves on a tree (chlorophyll). We are used to seeing these fade because pigments are affected by environmental and chemical conditions such as sunlight and oxygen. In contrast, you might find a dead beetle shell amongst dessicated autumnal leaves and see that it is as brightly iridescent as the day it was alive. This is because its iridescence is formed by structural colour rather than pigmentation. 

Structural colour is the result of nanoscopic structural arrangements on the surface of insect wings and some bird feathers. Because these are so tiny, they are able to interfere with visible light and scatter or reflect it back to produce intense colouration that does not fade. 

Two types of structural colour exist. One involves ordered, crystalline surface structures which look different when viewed from various angles and so to the human eye, appear iridescent. The other, modelled in the Cambridge study, is matt structural colour which has disordered structures which look the same from any angle. The colour resulting from the matt structures appears the most saturated and intense, and these structures seem less able to produce colours towards the red end of the visible light spectrum. This is why blues and greens in nature can be more vibrant, whereas the appearance of bright red is mostly due to pigments. 

Vincent Van Gogh’s 1888 View of Arles with Irises. The irises in the painting were originally painted using a mix of blue and red paint to create what used to be a violet shade. Now the red has faded so that only the blue remains. Image credit: Artwork by Vincent Van Gogh and photography by Finoskov, via Wikimedia, CC-BY-SA 4.0.

The researchers hoped that these colour limitations could be circumvented in the future to allow the production of commercial paints and dyes with structural colour rather than traditional pigments and dyes, which can be toxic and environmentally unfriendly. This would make structural colour paint more suitable for use in the home, but also as colour for products and labels because of its saturated appearance and resistance to fading. In the art world, painters would be able to rely on this type of paint to stay vibrant for decades, even centuries, to preserve their original image. The red component of the paint Van Gogh used for the purple irises in his 1888 work View of Arles with Irises has faded so much over the years that the flowers now appear blue. The researchers propose that a solution may be to use different types of nanostructures which might be more amenable to producing bright matt reds. However, more work needs to be done to understand these structures before we can expect fade-resistant crimson, cerise or carmine adorning the walls of our homes.

Written by Ellie Bennett and edited by Ailie McWhinnie.

Ellie Bennett is a Medical Law and Ethics masters student with a background in Biological Sciences. Find her on Twitter @ellieb2302 and Linkedin @Eleanor Bennett

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